1. Field of the Invention
This invention relates to waveguides for transmitting infrared radiation, and more particularly to a flexible hollow-fiber waveguide for transmitting mid-infrared radiation with low attenuation.
2. Description of Related Art
Radiation from visible, near-ultraviolet, and near-infrared lasers is transmitted well by simple, robust, inexpensive, nontoxic, solid oxide-glass fibers. However, a problem persists in devising a delivery system for transporting radiation from a mid-infrared laser to the point where application of that radiation is desired. Laser emissions at these wavelengths are not transmitted well by most solids. Whether the use be industrial or surgical, a satisfactory delivery system for mid-infrared should: (1) allow focal-spot sizes under 0.5 mm (preferably down to 0.1 mm) in diameter, at a reasonable working distance for the particular use, without requiring a large final optic; (2) transmit at least several tens of watts of average power, with low attenuation; (3) be easily maneuvered about an application site, with little resistance to motion and with minimal bulk to impair access to and viewing of the application site, and without substantial change in the output characteristics; and (4) be simple, robust, and relatively inexpensive. For surgical applications, the delivery system also must not itself be hazardous to the patient (such as are fibers containing toxic materials that are soluble in body fluids). It will be appreciated by those skilled in the art of laser application, that the requirements for good focusability and for unchanging output characteristics necessarily imply that the laser operate in the Gaussian or TEM.sub.oo mode and that the delivery system not substantially degrade that mode. For the laser wavelengths of immediate present interest (roughly, 2.5-12 .mu.m), hitherto there has been no satisfactory delivery system. Articulated arms provide good beam quality and good transmission, but are awkward, complex, and expensive, and require frequent realignment. Solid fibers for this wavelength region provide poor beam quality and only fair transmission, are generally toxic, are expensive, and can be bent only a very limited number of times and to a rather limited degree. Accordingly, the art has attempted to use hollow waveguide fibers.
Prior-art hollow n&gt;1 waveguides provide poor beam quality (poor transverse spatial coherence). FIG. 12B of Gregory & Harrington, and FIGS. 5 and 6 of Croitoru et al., "Characterization of hollow fibers for the transmission of infrared radiation", Appl. Opt. v. 29, 1805-1809 (20 Apr. 1990) and Dror et al., "Hollow Tubes for Transmitting IR Laser Energy for Surgery Applications", presented to ICALEO '89 (15-20 Jan., Los Angeles), are representative of the characteristics of prior art hollow n&gt;1 waveguides. Such waveguides have only fair transmission. Typically, waveguides such as are disclosed in Matsuura & Miyagi, "Low-loss metallic hollow waveguides coated with durable and nontoxic ZnS", Appl. Phys. Lett. v. 61, 1622-1623 (5 Oct. 1992) are superior. Waveguides with metal tube walls which serve as the supporting structure for any coatings (such as disclosed in U.S. Pat. No. 5,005,944, issued to Laakman et al., and U.S. Pat. No. 4,913,505, issued to Levy '505) may be capable of handling substantial power, but are semiflexible at best. Those with plastic tube walls (such as disclosed in U.S. Pat. No. 4,930,863, issued to Croitoru et al.) are flexible, but have marginal power-handling capability at best, high loss, and lack coherence. As such they are suitable principally for signal-handling. Yet-earlier devices such as disclosed in U.S. Pat. No. 3,436,141, issued to Comte, U.S. Pat. No. 3,583,786, issued to Marcatili, and U.S. Pat. No. 3,963,828, issued to Onoda et al., have not proven useful for the applications of present interest at the wavelengths of present interest.
Hollow waveguide fibers having an index of refraction less than one, have not yet attained both transmission characteristics and flexibility required for many applications. However, in other respects these waveguide fibers are quite satisfactory at selected wavelengths. See Gregory & Harrington, "Attenuation, modal, and polarization properties of n&lt;1, hollow dielectric waveguides", Appl. Opt. v. 32, 5302-5309 (20 Sept. 1993).
The present invention provides a waveguide, and method for making such waveguide, that meets the need for a flexible mid-infrared laser transmission medium which has relatively low loss and transmits the laser radiation without disruption of the TEM.sub.oo mode.